Alex Lanstein, a security analyst at FireEye Malware Intelligence Labs, who worked with Microsoft on the successful takedown operation, credits the tactic as being the main reason why Rustock stayed in operation for more than five years.

Rustock, which spread largely via drive-by download from compromised websites, turned compromised machines into spam-spewing monsters. At its peak, the botnet was responsible for almost the worldwide production of spam. Much of this junk mail promoted sites selling unlicensed pharmaceutical drugs, such as Viagra. With the closure of the world's largest spam affiliate program – spamit.com – back in October, this volume of junk mail dropped slightly. This was essentially because one of the botnet owner's main clients had shut up shop, but Rustock was still responsible for at least 30 per cent of global spam.

The dismantling of the core network of around 26 servers that were used to control infected clients meant that this spam torrent suddenly trickled away to nothing last Wednesday.

Rustock: My part in its downfall

The operation to mount the takedown began nine months previously, when Microsoft held a conference call with security partners. During the call, Rustock emerged as one of the primary threats facing Redmond's customers. A loose-knit committee focusing on dismantling Rustock was formed which, over time, divided itself into four organisations as other security researchers dropped out to tackle other projects. The four groups included representatives from Microsoft itself, along with staff from pharmaceutical giant Pfizer, as well as security researchers from FireEye and computer scientists from the University of Washington.

Lanstein, who had previously worked on the takedown of the Mega-D and Szerbi botnets, told El Reg that the pooling of resources was necessary because Rustock was a more sophisticated threat and everyone had only part of the overall jigsaw. "Everyone learned from each other," he explained.

Compromised machines were infected with malware packed via custom encryption techniques that made it look like a .rar compressed archive file. Each machine communicated with control nodes via posts that, to a casual observer, would look like posts to a bulletin board or web forum. In addition, the command and control servers were all US-based and mostly located with small hosting firms who had no idea that anything was amiss... and not the specialist cybercrime-friendly hosting facilities in eastern Europe beloved by most cybercrime gangs.

Using this approach, the Rustock botnet herders were largely able to keep their systems off the radar of collaborative anti-spam and net security efforts run by the likes of Spamhaus and Shadowserver.

FireEye was able to fingerprint what communication between zombie clients and the command nodes of the Rustock network looked like. In this way it was able to identify the main command nodes and backup servers used by Rustock.

Microsoft is operating sinkhole servers for Rustock command and control domains so they are able to identify PCs compromised with botnet software, providing more useful raw intelligence on the zombie network in the process.